But there is a reason for all that variance. Decades ago, there was a standard in sizing — a time when a medium was a medium, no matter what — but that faded as the market became more brand-centric and each brand developed its own sizing system, Murphy explained.

"What's happened over time is that (brands) have evolved their sizing to represent who they believe their core customer is," she said. "That's why we have so much inconsistency. If you have a brand catering to a 60-year-old customer, their size medium is going to be representative of their demographic. If you contrast that with a brand serving a tween or teenage customer, their size medium is going to look very different."

These cases aren't flukes. Research from True Fit shows the actual waistband measurement of a pair of women's size 6 or 28 jeans can vary by more than 5 inches, depending on the brand. (That variance increases when grouping together different rises, from low to high.)

And it’s no secret that many brands engage in so-called "vanity sizing," in which they make the clothes bigger than what the size on the tag suggests. So, for example, a size 4 really fits like a size 6. The thinking behind the tactic is that women are presumably more likely to buy an item when it makes them feel good about themselves.

Research published in The Washington Post last year illustrated how a size 8 dress today is about the same as a size 16 dress in 1958.

Not being able to find the right size is largely a women’s problem, especially since many men’s garments are sold by measurements. Most women wear about three different sizes, while men typically go between two sizes, Murphy said. And that likely influences return rates, according to research from True Fit.

Women's sizing retail statisticsAngeliki Jackson / TODAY

Looking at $40 billion in online transactions across multi-brand retailers — where return rates are higher, because customers are shopping across many brands and also can’t try the clothes on first — the company found that most of the time, it’s women sending the clothes back.